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ABSTRACT

The installation cost of a ground-source heat pump system can be minimized by optimizing the length of the ground heat exchanger. For a given system, the length depends, among other factors, on the pipe thermal conductivity, which can be increased by mixing additives to the polymer resin used to extrude the pipe. Using this method, IPL has manufactured a new high-density polyethylene pipe whose thermal conductivity is 0.7 W·m^-1·K^-1 (0.40 Btuh^-1·ft^-1·°F^-1), which is 75% higher than that of regular high-density polyethylene. Two-and three-dimensional numerical simulations were used to evaluate the performance of the thermally enhanced pipe in vertical ground heat exchangers used with ground-coupled heat pumps. The borehole thermal resistance and the water temperature inside the pipes during heat exchange were evaluated numerically. Simulations show that the thermally enhanced pipe reduces the borehole thermal resistance by up to 24% for ground heat exchangers made with a single U-bend, which can, in turn, shorten the required borehole length. The water temperatures for equivalent heat injection or extraction are also decreased and increased, respectively, with the thermally enhanced pipe, which therefore enhances heat pump performances.